Spectrometers are widely used in both research and industry for analysis, detection, and confirmation of material composition. In recent years, handheld spectrometers operating in the visible, near infrared, and mid infrared spectral ranges have been prolific and been sold into numerous markets.
Spectrometers are used to analyze materials often in order to determine elemental or molecular composition. The more wavelength resolution and/or spectral bandwidth, i.e. wavelength range that a spectrometer is capable of, the more compositional information can be attained which in turn leads to the ability to analyze more complex materials and mixtures. Another important aspect of small handheld spectrometers is measurement speed. Users are typically able to hold a portable spectrometer up to a sample for seconds and even up to a minute, but much longer than this leads to fatigue and the inability to measure many samples quickly. Yet another important characteristic of portable spectrometers is overall cost. Finally, and perhaps most important for portable spectrometers, is instrument reliability and overall ruggedness for field use.
One common portable spectrometer design employed by Ocean Optics, Stellar Net, Avantes, Thermo Fisher Scientific and multiple other vendors for the deep UV to near infrared wavelength range (200 to 1100 nm) utilizes inexpensive silicon based CCD linear array detectors. The design eliminates the need for the older scanning gratings and basically dedicates each pixel in the linear array detector to a particular small wavelength range. The advantage is no moving parts, small size, ruggedness, and low cost. These spectrometers typically have several thousand pixels and can be designed for a small slice of the UV-VIS-NIR spectrum with high resolution or a larger slice with lower resolution.
Similar spectrometers are also available at longer wavelengths in the Near Infrared (NIR) beyond where silicon detectors function. These spectrometer (offered by the same vendors previously mentioned) typically utilize Indium Gallium Arsenide (InGaAs) detector arrays. While these function well, the disadvantage is that they are extremely expensive in comparison to silicon arrays and depending on desired performance and they usually require cooling. The cooling adds to volume and power consumption making these devices less portable.
An optional NIR portable spectrometer configuration is to use a single small element InGaAs detector in conjunction with a MEMS light modulator. Polychromix successfully introduced such a portable spectrometer in 2006. Similar designs can be found in the literature using Texas Instrument's digital micro-mirrors as well. See also U.S. published application No. 2008/0174777 incorporated herein by this reference.
There is currently not a large selection of portable spectrometers with both a wide spectral range and high resolution on the market. Spectral Evolution and ASD offer reasonably high resolution spectrometers that cover the wide range of roughly 400 to 2500 nm but do so with the added drawbacks of size, weight, and expense. Internally, these units consist of multiple versions of the spectrometers previously described.
An optional design for high resolution wide spectral range spectrometers is the Echelle spectrometer. These spectrometers use a two dimensional detector and a cross dispersing element (such as a prism) in addition to the usual diffraction grating to spread spectrum in two dimensions across both dimensions of the detector. A limiting factor of this type of design remains that the currently available detectors cannot cover the wide range from 400 to 2500 nm. For example, silicon detectors always lose sensitivity above 1100 nm. While it is feasible for certain InGaAs detectors to work over nearly this entire range, the cost is even more prohibitive that that of the linear (1D) InGaAs arrays.